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Thermodynamic and economic assessment of off-grid portable cooling systems with energy storage for emergency areas
Accepted Manuscript Research Paper Thermodynamic and economic assessment of off-grid portable cooling systems with energy storage for emergency areas Hasan Ozcan, Umit Deniz Akyavuz PII: DOI: Reference:
S1359-4311(16)34522-7 http://dx.doi.org/10.1016/j.applthermaleng.2017.03.046 ATE 10050
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
31 December 2016 6 March 2017 10 March 2017
Please cite this article as: H. Ozcan, U.D. Akyavuz, Thermodynamic and economic assessment of off-grid portable cooling systems with energy storage for emergency areas, Applied Thermal Engineering (2017), doi: http:// dx.doi.org/10.1016/j.applthermaleng.2017.03.046
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THERMODYNAMIC AND ECONOMIC ASSESSMENT OF OFF-GRID PORTABLE COOLING SYSTEMS WITH ENERGY STORAGE FOR EMERGENCY AREAS
1*
1
Hasan Ozcan, 2 Umit Deniz Akyavuz
Karabuk University, Engineering Faculty, Mechanical Engineering Department, Baliklar Kayasi Mevkii Demir Celik Kampusu, Karabuk, 78050, Turkey
2
Kilis 7 Aralık University, Kilis Vocational College, Electricity and Energy Department, Kilis, 79000, Turkey * E-mail: [email protected]
ABSTRACT This study aims to investigate performance and cost aspects of a solar powered portable cooling system to conserve first aid supplies for off-grid areas with energy storage. Due to the intermittent nature of solar energy availability, two energy storage options are considered for a stationary system. Additional to the standalone system without energy storage, hydrogen is selected to be the storage medium by considering electrolysis at day time, and use of a hydrogen fuel cell unit at night time. This system consists of solar photovoltaic cells, a Polymer Exchange Membrane (PEM) electrolysis unit (PEME), hydrogen tank, a PEM fuel cell unit (PEMFC), and a vapor compression refrigeration (VCR) system to condition a container rated with ~11 kW cooling load. The second system utilizes pumped – hydro storage (PHS) technology using a simple pump – turbine couple by storing water at a higher reservoir during day time and utilizing it to produce hydro power at night. Existence of higher reservoir brings a significant additional cost for the PHS system, making this configuration almost four times more costly than that of the hydrogen storage option, even though the storage efficiency of the PHS system is significantly higher than the hydrogen storage.
Keywords: Emergency areas, Syria, PEM electrolyser, PEM fuel cell, pumped-hydro storage, air conditioning,
1. Introduction Poverty, scarcity, starvation, infectious and epidemical diseases, and wars interrupt daily routines of communities which cause physical, economical, and social losses due to unstable natural or human based disasters [1]. It is of importance and crucial to have ready to act supplies during and after any emergency situation, where the primary services are first aid, feeding, and electricity, respectively [2]. For regions where
1
access to electricity is impossible, either a fast transportation of medical supplies are required, or a conditioned storage medium should be present for fast intervention to emergency situations. Storage conditions of medical and pharmaceutical supplies are specific to many factors. Most medical supplies have to be conditioned in a cold chain, to protect the products from losing their curing properties. Especially vaccines should not be interacted to undesired temperatures longer than an hour [3]. Temperature ranges of most vaccines and other medical supplies are suggested to be kept between 2°C - 8°C [4]. Storage conditions of some vaccines are provided in Table 1.
Refrigeration systems require a main source of electrical work in order to energize the system compressor for pressurized working fluid to complete the cyclic process. Solar energy is one of the best solutions for off-grid energy supply with its abundancy in many regions in the world, while solar photovoltaics (PV) has been one of the most promising renewable options to generate direct electricity at off-grid regions, which can be simply defined as a cell produced by thin semi-conductive junctions [5], leading to generation of direct electricity by many parallel or serial connected cells. The intermittent nature of solar energy brings the problem of steady energy generation which is generally solved by battery storage [6]. The most efficient battery storage technology is the Li-Ion storage with high efficiencies. Several researchers have imposed on specifically the environmental concerns on production and disposal of batteries, as well as their low life cycles, and daily self-discharging problems [7].
As alternatives to battery packs, electrolyser – fuel cell units are extensively under development stage. These storage systems show superiority than battery systems in many aspects, however, low round efficiency and high capital costs due to their industrial immaturity make this option a less feasible one. Cleanliness of both systems are also under further investigation [8]. Another option for energy storage can be considered as pumped-hydro storage, which is based on the simple idea of pumping water at a higher reservoir when there is available electricity, and utilizing the potential energy using a hydraulic turbine at night time [9].
Studies on PV based hydrogen production is comprehensively reviewed by Abdin et al, resulting in proposed PV based hydrogen storage systems with promising potentials, even though a high capital cost is required [10]. Other studies are performed by Aris and Shabani, on sustainability of PV-fuel cell couples [11], and Chowdhury and Moursed, on off-grid solar houses, their concepts, and long term advantages [12]. A portable solar PVdesalination system is recently introduced by Beitelmal and Fabris [13]. Many other studies on thermodynamic and economic aspects of such configurations can be found elsewhere [14-17].
2
Mathematical modeling and working principles of a PHS system using a wind-solar hybrid energy as the source is proposed by Ma et al, claiming that a 100% energy independency is attainable with such system configuration [18]. In another study a cost optimization of the aforementioned PHS system is performed using Genetic algorithm as an optimization tool by taking into account the number of PV modules, pump and reservoir dimensions [19]. Safety and economic criteria of a designed PHS system for a remote island are discussed by Ma et al for the main components and optimization comparsions [20]. A comprehensive review of several applied PHS systems and their feasibilities are reported in a recent literature survey performed by Sontake and Kalamkar [21]
In this study, a solar PV based refrigeration system with hydrogen storage and pumped-hydro storage are considered for a fixed Evergreen cooling container sized with a ~11 kW cooling load [22]; a comparative thermodynamic analysis is performed with parametric optimization, a case study is conducted by considering the solar data for Aleppo as an emergency region [23], and a rough estimation on system costs is made by considering the thermodynamic data obtained for system components. Studied systems with storage are also compared with the conventional solar VCR system in order to discuss on the thermodynamic and cost aspects of energy storage to provide a mobile and stationary system.
2. Systems Description A standalone solar VCR system as represented in Fig 1 is initially analyzed as the reference model. PV panels are used as the DC electricity supplying source for the compressor of the VCR system, where inverters are used to convert DC into AC. DC powered cooling systems are also available that directly utilizes the DC in the system compressor, resulting in enhanced cooling system performances. However, this study focuses on more conventional cooling systems with compressors driven by AC electricity. Therefore, inverters are present fro both PV panels and the PEMFC unit. Evaporator temperature is taken as a constant input parameter considering the medical supplies’ conservation temperature range, where the condensation temperature is based on the pressure ratio assumption made for the cycle. Here, a down selection process for the working fluid is also taken into account by making comparison for various refrigerants that match with the cooling temperature required for medical supply conservation. For this purpose, R134a, R152a, R600a, R12, R22, R290, R410a and R1234yf refrigerants are selected as candidate refrigerants, the highest efficient is selected, and adapted to other configurations with energy storage to provide performance enhanced systems.
3
The second system (S-H2-VCR) consists of solar PV panels connected to the VCR system along with the electrolysis unit. Electricity produced from PV panels are used for both the VCR and electrolyser unit at day time, while the generated hydrogen from the PEME is stored in a hydrogen tank to be utilized in the PEMFC unit when PV panels do not produce electricity. Here, oxygen is also considered to be stored to be a by-product which might also be useful as a medical consumable. This configuration makes the solar VCR a steadily working system. Schematics of the system is provided in Fig 2. The third system (S-PHS-VCR) differs from the second one in terms of the energy storage method. Here, instead of using an electrolyser – fuel cell couple, a hydraulic pump-turbine couple are considered to supply water to a higher reservoir to store it to be utilized at night time to produce power as shown in Fig 3. Here, generated electricity from PV panels is now both provided to the VCR and the hydraulic pump unit to provide potential energy to the water which is to be stored at a higher spheroid reservoir. This simple method of energy storage is expected to be high efficient with it maturity, however, it is also expected that the large sizes of required reservoirs might jeopardize the portability of the solar VCR unit.
3. Analyses and Assessment In this section, thermodynamic and economic analyses of the studied systems are represented with necessary assumptions and input parameters in subsections. 3.1. VCR subsystem Compressor power requirement, evaporator cooling load, and condenser heat release are defined by writing energy balances for individual components of the VCR subsystem as follows:
(1)
Where W and Q stand for work and heat. The thermal exergy of the container is
(2)
Finally the energetic and exergetic coefficient of performances (COP) of the VCR system are
(3)
4
3.2. Solar PV subsystem The load current is defined related to the voltage at constant temperature and solar radiation as follows [24]: / 1
(4)
Here, Io is the dark current, e is the electron load, k is the Boltzmann constant and T is the absolute PV cell temperature in Kelvins. Rearranging by equating the Eq (4) to zero, the definition becomes
/ 1 or
1
(5)
Here Voc is the open circuit voltage. The power in the PV cell is
exp 1"
(6)
The maximum power is obtained by equating the derivative of Eq (5) to zero as: 1
1
(7)
Where Vm is the voltage at the maximum power. Rearranging Eqs (4) and (7) one obtains the load current corresponding to the unit cell area as: /
/#
/
/#
(8)
and the maximum power per unit area as; /#
! "
/#
(9)
Open and short circuits are extreme cases and the power delivered to the load is zero. The power is at its peak at only for a specific load, which is dependent on the optimal current density and the cell voltage. The maximum energy efficiency attainable from the PV panel is the ratio of the maximum power delivered to the total solar energy input $
#
(10)
Where In is the total solar irradiation delivered on the total PV surface (In=IA*Ac). The solar exergy input to the PV panels are as follows [37]: $ 1
" #
(11)
Here, % is the reference temperature while & is the sun temperature taken is 5700 K [25]. Since this equation involves many peculiarities of thermal radiation such as irreversibility of emission and absorption of radiation, significance of thermal radiation exergy for a zero environment temperature, exergy at a varying environment temperature, and the analogy between exergy of substance and radiation as mentioned in [37], it is selected to be
5
the main solar exergy input for the present study. The exergy efficiency of the PV subsystem is the ratio of the maximum electricity produced to the solar exergy input as follows: %
''
(12)
3.3. Hydrogen Storage Subsystem 3.3.1 Electrolyser The thermo-neutral voltage transfers the reaction enthalpy to the adiabatic cell and defined as follows [26]: )
∆+
(13)
'
The reversible cell voltage (Ecell) is the potential applied on the cell and it is the required free energy to conduct the electrochemical reaction and can be explained by the Nernst equation. In this equation, the standard cell voltage is (E0) is 1.229 V, the equilibrium constant is &, '+ -%./ (/')+ - %./ (. The reversible cell voltage is
∆1
,$
'
2 '
ln &,
(14)
Current density of the electrolysis cell is the ratio of the current to the cell area and dependent on the activation energy '∆. ,$ 20 ,$ ( based on the Butler-Volmer equation 3 3
exp
4' , 2
exp
54 ' , 2
(15)
Here, 1 is the transfer coefficient varying from 0 to 1, and the exchange current density 2% is 2% 304
' , 2
(16)
Total electrolysis voltage ( ,$ ) is the summation of the reversible cell voltage along with the activation and concentration overpotentials ,$ ,$ ,$ ,$ ),$
(17)
The activation, concentration and ohmic overpotentials are defined as follows: 2
3
'
3
,$ , at 2 5 2% (18) 3
,$ 6 , at 2 7 2% 3
,$
5
2
8
6'
9
(19)
9
),$ 2 :%
(20)
78
6
Where b is the Tafel constant, JL is the limiting current density, ; is ionic conductivity through cell, and < is water content ar the membrane-electrode interface. Finally, energy and exergy efficiencies of this unit can be defined by considering the higher heating value (HHV) of hydrogen produced as the useful output $
:.;:
(21)
,
Here there is a constant relationship between the exergy content of hydrogen and it is heating value, therefore, a relation can be made between the exergy and the energy efficiencies as follows: % 0.83$
(22)
3.3.2 Fuel Cell For the PEM fuel cell, the reversible voltage across the load can be defined by taking the overpotentials into account as follows: $ )
(23)
Here, it is possible to express the voltage across the load (Eload) in terms of the reversible cell voltage and overpotentials through the cell as follows [27]: $ 1.229 8.5 105 298.15 4.3085 105/ ' + 0.5 - (
4 4 2 4 4
6'
3 <
2 D 3
4 4 2 4 4
6'
3 =>!5.?/
4 4 2 4 4
6'
3
3
"
%.%%/ ;@5%.%% :
(24)
Here, the first term is the open circuit voltage (1.229 V), second term is present for the effect of the cell temperature, the third term is correlated to represent the effects of both cell temperature and pressure, the remaining terms represent the activation, concentration, and the ohmic overpotentials, respectively. Here, it is assumed that the transfer coefficient is taken as 0.5 for the anode and 1 for the cathode, the number of charges are z=2, and the limiting current density is 2000 A/m2. Many other input parameters and their numerical values are provided in Table 2. The exchange current density of the cell is 2% 1.08 0.086A
(25)
Where Tb is the boundary temperature, the concentration overpotential exponent F is around 2, and the concentration overpotential factor and the membrane humidity factor are defined, respectively, as follows: DG
7.16 105 A 0.622 1.45 105 A 1.68 8.66 105 A 0.068 1.60 105 A 0.54
0 N ) N 1L 0.043 17.8) 39.85) 39.85) MG 14 1.4) 1
1I)N3
I 2 )JL K 2 )J
(26)
(27) 7
Here the activity coefficient of the water is ) O+ - / where P is the membrane pressure and O+ - is molar fraction of water. The cathode pressure is correlated using 8.52- equation. Assuming there is linear exchange from the molar fraction from anode to cathode, partial pressures of hydrogen and oxygen are given as (+ O+ B ) and (- O- C ), based on the anode and cathode pressures. It is possible to perform a molar fraction calculation for the fuel O+ , and oxidant O- as documented in [27]. The power output from the fuel cell stack is related to the fuel utilization factor (Uf), and amount of feed fuel ( ), as a product of actual voltage as follows: 'C 2 0 P $
(28)
Energy and exergy efficiencies of the fuel cell stack are defined as the ratios of useful outputs per inputs, respectively, as follows [28,29]: $'C %'C
(29)
9+
(30)
3.3.3 Hydrogen Tank Since it is expected that the fuel cell unit produces a constant amount of power to drive the VCR system compressor, it is possible to determine the amount of hydrogen required to run this process at night time. Here the required amount of hydrogen can be found using Eq (27), and the stored hydrogen is calculated using this input as follows: Q, 14 3600 + 2
(31)
Here the numbers 14, 3600, and 2 correspond to the nigh time hours, seconds of an hour, and the molecular weight of hydrogen, respectively. This simple equation allows to estimate the size of the hydrogen tank for storage, which is important to know for a portable system. The volume of the tank can be calculated by using the specific volume of hydrogen at its storage temperature and pressure as follows: Q, R
(32)
3.4. PHS system Conversion of electrical - potential energy principle is the basics of the PHS system using two simple mechanisms to circulate water between higher and lower reservoirs. Through the analysis, coefficients for water pumping, S> and turbine power production, S are two important parameters, indicating the water volume stored by unit energy. [30, 31]. The suction volume flow rate of the pump from the lower reservoir is T> J
D E FG)
S> EH> J
(33) 8
Here $> is pump efficiency, U is water density, V is gravitational force, is the higher reservoir height, and EH> J is the power delivered from the PV panels. The turbine power is defined as: J $ UV T J S T J
(34)
where $ is turbine efficiency and, T J is the volume flow rate of water entering the turbine. The total water stored at the higher reservoir can be defined with its potential energy content as follows [32]: > , Gü $
D FG)
(35)
,:J%!
3.5. Economic Analysis Since the corresponding portable cooling system is specifically designed as a humanitarian application, a rough cost estimation is considered by using the thermodynamic and cost data of all components throughout the systems studied. Table 3 summarizes correlations for component costs with additional comments on individual equations. Total system costs are the summation of component cost with a 1.1 of auxiliary factor and defined for each system as follows: $,KLK E #PW
(36)
$,KLK E $ 'C #PW
(37)
$,KLK 'E > & ( #PW
(38)
All aforementioned cost parameters are updated using the CEPCI (Chemical Engineering Plant Cost Index) in order to provide up-to-date component costs.
4. Results and Discussion 4.1. Thermodynamic analysis results In this study, a thermodynamic analysis and economic assessment of three solar cooling applications are comparatively performed. Parametric optimization of all three systems are conducted using Engineering Equation Solver (EES) is utilized for analyses. Here, many input parameters as represented in Table 2 are used as variables for optimized system configurations. Results of thermodynamic and electrochemical models used for analysis are manipulated using various input data in order to accomplish higher performing and lower cost systems. Various representations of parametric optimizations are given in this section, visualizing the changes in system performances by varying several input parameters.
An initial modeling of the system is performed without considering the storage option in order to parametrically optimize the refrigeration system by taking into account many refrigerants showing similar thermophysical 9
properties as R134a. This working fluid is compared to R600, R1234yf, R600a, and R152 fluids. A down selection is made by eliminating R410a due to low exergy efficiency values of the overall system compared to those of other considered fluids as represented in Fig 4. Total exergy destruction rate shows its lowest value for the R152a fluid, which is slightly lower than R134a, while other selected fluids show higher exergy destruction rates at same conditions as shown in Fig 5. It is also obvious that the system tends to show lower exergy efficiencies at higher solar radiation. This due to the definition of solar exergy input.
R152a is selected as the working fluid for the systems with storage, in which the energetic COP values are 5.99 for ideal conditions. For the 11 kW container, the power requirement of the compressor is 2 kW. Therefore, sequence of the modeling is initiated with the PEMFC stack with a known power output. The current density affects all considered overpotentials through the FC with a similar trend as shown in Fig. 6. Increased current density leads to higher overpotentials, however, increase in overpotentials show a decreasing trend leading to higher FC voltage (~0.49 V). Since the required power is known, it is possible to determine the required molar hydrogen rate for the FC stack. 0.022 moles/s of H2 is required to produce enough power to run the refrigeration system. Energy and exergy efficiencies of the FC stack are determined to be 33%, and 39.6%, respectively.
Considering the required molar flow rate of H2, one can determine the total daily H2 production requirement of the electrolysis cell. For 10 hours of sunbathing 2.27 kg of H2 should be stored in daily basis. Since both FC and electrolysis cells assumed to work at same pressure to prevent from losses due to expansion or compression of gas agents, H2 storage is considered to be at the same pressure with both cells, namely 2500 kPa, and at this pressure level, specific volume of H2 is ~0.5 m3/kg. This brings a H2 tank volume requirement of 1.15 m3, which is in a feasible range for the considered container cooling load. A parametric study is conducted in Fig. 7 to evaluate effects of fuel utilization factor and cell temperature of the FC stack on the hydrogen tank volume requirement. Higher utilization ratio and cell temperature values result in higher tank volumes, even though these parameters are favorable for higher FC stack energy and exergy performances. H2 discharge lasts in 14 hours, however, charging of H2 should be made in 10 hours of sunbathing while PV system produces electricity. Therefore, produced hydrogen rate should be higher than the consumed hydrogen at a rate of 0.0315 mole/s. In order to produce this amount, the power requirement of the electrolysis unit is determined to be 11.13 kW. Another parametric optimization is conducted for a decreased power requirement of the electrolysis cell by
10
manipulating the FC fuel utilization factor as in Fig. 8. Higher fuel utilization factor significantly affects the power requirement of the electrolysis unit, due to FC H2 requirement significantly decreases.
Using the simple definitions for a rough model of the PHS system, the higher reservoir conditions should be taken into account for a parametric optimization. Reservoir height and the spherical type reservoir dimeter and volume are the primary cost influencing parameters. However, change in these variables also affect the storage efficiency and the required solar panel area to provide power for the hydraulic pump. Higher reservoir height results in decreased storage efficiency and increased pump power requirement, and higher amount solar PV panels as represented in Figs 9a and 9b. Even though higher height does not seem to be feasible both thermodynamically and economically, a higher volume of the reservoir would bring a very bulky component, which possibly jeopardize the portability of the system. Considering the reservoir height as a variable input, the total PHS efficiency shows a significant energy and exergy efficiency value of 74% at the lowest height value, while decreasing down to 54% at higher reservoir height. It should also be noted that, since the useful output and required input of the PHS system are related to power, the energy and exergy efficiencies of the PHS system are equal.
The PV panels are required to supply enough power for both the electrolysis unit and the refrigeration system, Therefore sizing of the panel area is based on the sum of these power requirements. The total PV area required to run this S-H2-VCR system is found to be around 105 m2, for the PHS system required PV area is around 46 m2, while this value is only 16.7 m2 for the standalone system without storage. Energy and exergy efficiencies of the PV panels are determined to be 25.2% and 27.5% respectively, consistent for all configurations. Considering the overall system performances for the standalone system, S-H2-VCR, and S-PHS-VCR systems, energetic COP values of studied systems are 1.39, 0.211, and 1.34, while exergy efficiencies are 11.54%, 1.75%, and 11.15%, respectively. Low values for the second system are due to the definition of overall system efficiencies in which generated H2 is not considered as an output from the system.
Considering the monthly solar irradiation data [23] for Aleppo-Syria as an emergency region COP and exergy efficiency values at each month of the year are provided in Figs 10a and 10b. Since the solar irradiation value is the major input for both systems, months with lower irradiations provide the highest efficiencies. With the two major assumptions made for this city, namely ten hours of sunbathing which corresponds to the lowest daylight
11
time during the year in Aleppo, and the lowest possible irradiation values, both systems with storage might safely produce conditioned air for medical supplies throughout the year.
4.2. Economic analysis results Table 4 summarizes all component costs of all three systems considered. The highest cost belongs to the higher reservoir of the S-PHS-VCR system, corresponding to almost ~80% of the total system cost. Since the required panel area changes for every system, costs are also variable. Share of solar panel cost shows its highest value for the S-H2-VCR system due to higher solar panel area requirement for H2 production during day time, occupying around 60% of the total cost of this system. Cost shares of components of each studied system are represented in Figs 11a, 11b and 11c. Even though the thermodynamic performance of the S-PHS-VCR system is significantly higher than that of the S-H2-VCR system, high reservoir cost and a predicted bulky size of the reservoir make such system costly while jeopardizing the portability of such system. However, it might still be possible to transport such reservoir for this 11 kW system considered. Higher PV area requirement of the S-H2-VCR system might also be considered as a problem for a portable system, however, recent technologies for light, foldable, and cheaper PV cell design make this configuration a promising one for a feasible stationary and portable cooling container.
Comparing both systems with the battery storage technology, a simple assessment can be made for battery costs with their life cycles. For the 2 kW peak-power requiring air conditioning system, and 16 hours of stored power utilization requirement at night, the total energy to be storage by the battery packages are to be 32 kW/h. In order to possess a reasonable life cycle, the discharge rate of each battery should not be lower than 50%, which may increase the life cycle of a battery package up to 6 years. Therefore, this simple approach suggests at least a 64 kW/h battery storage for the studied system [38]. Considering the commonly used Li-ion battery systems, updated costs are around 150 kW/h, cost of the battery storage is roughly expected to be $9600. Even though low life cycles, maintenance requirements, and possible hazardous disposal of these battery systems are seems to be infeasible, its cost is significantly lower than that of the PHS and similar to the S-H2-VCR system.
5. Conclusions
12
Present study thermodynamically and economically evaluates a solar based refrigeration system with hydrogen and pumped-hydro storage to keep medical supplies at their desired temperature ranges for off-grid areas with a case for Aleppo-Syria. The main motivation is to store medical supplies at their desired storage temperatures. Most medical supplies, especially vaccines have to be conditioned in a cold chain, to protect supplies from losing their curing properties. If vaccines are interacted to the environmental temperatures more than an hour, the weak viral organisms may either die or get strong, making both situations hazardous for human body functioning when injected. The reported energy and exergy efficiencies, as well as the rough cost estimation of studied systems are in feasible ranges and all studied sub-systems relatively possess high life cycles with acceptable maturity. Hydrogen storage shows superiority with low cost and a light - small size storage capability than that of the pumped-hydro storage option, while the pumped-hydro storage shows significantly higher energy and exergy performances. Estimated cost of the PHS option almost quadruples the H2 storage option, even though performance characteristics of PHS are significantly higher. Battery storage can also be another option with possible low life cycles and high round efficiencies. Such configurations can conveniently be used for off-grid areas, not only for cooling, also for continuous electricity supply with mature and on-the-shelf equipment.
6. Acknowledgement Authors acknowledge the financial support from the scientific research projects unit of Karabuk University under the project number KBÜ-BAP-16/2-YL-091.
Nomenclature A
: Area
a
: Membrane water activity
C
: Cost
E
: Voltage
Ex
: Exergy
e
: Electron load
F
: Faraday constant
h
: Enthalpy
h, z
: height, charge
I
: Current
13
IT
: Solar irradiation
J
: Current density
k
: Boltzmann constant
P
: Pressure, power
Q
: Thermal energy
R
: Resistance, Universal gas constant
s
:Entropy
SF
: Safety factor
U
: Utilization, heat exchange factor
V
: Velocity
W
: Work
y
: molar fraction
Greek Symbols ??
: Exponent
η
: Energy efficiency
ψ
: Exergy efficiency
σ
: Membrane thickness
α
: Load transfer coefficient
γ
: Exponent for concentration overpotential
γ
: Molar fraction
β
: Facto for concentration overpotential
ω
: Membrane humidity factor
ρ
: Density
ΔT
: Temperature difference
Subscripts and Superscripts a
: Anode
act
: Activation
b
: Boundary
14
c
: Compressor
cat
: Cathode
cell
: Cell
ch
: Chemical
conc
: Concentration
cond
: Condenser
cont
: Container
D
: Destruction
elec
: Electrolyser
en
: Energetic
eva
: Evaporator
ex
: Exergetic
f
: fuel
gen
: generation
ev
: expansion valve
in
: inlet
is
: Isentropic
kin
: Kinetic
L
: Limit
m, max
: Maximum
nf
: Non-flow
o
: Reference
oc
: Open circuit
ohm
: Resistance
out
: output
ph
: Physical
pot
: Potential
pump
: Pump
ref
: Refrigerant
rev
: Reversible
15
res
: Reservoir
sat
: Saturation
t
: Turbine
tot
: Total
w
: water
Acronyms AC
: Alternative Current
CEPCI
: Chemical engineering plant cost index
DC
: Direct Current
PEME
: Polymer exchange membrane electrolysis
PEMFC
: Polymer exchange membrane fuel cell
PHS
: Pumped-hydro storage
PR
: Pressure ratio
PV
: Photovoltaics
S
: Solar
VCR
: Vapor compression refrigeration
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[15] Cordiner S, Mulone V, Giordani A, Savino M, Tomarchio G, Malkow, T, Tsotridis G, Pilenga A, Karlsen ML, Jensen J, 2016. Fuel cell based Hybrid Renewable Energy Systems for off-grid telecom stations: Data analysis from on field demonstration tests. Applied Energy, in press, corrected proof.
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[17] Janko S, Atkinson S, Johnson N, 2016. Design and Fabrication of a Containerized Micro-Grid for Disaster Relief and Off-Grid Applications. In ASME 2016 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference 2016; V02AT03A056-V02AT03A056.
[18] Ma T, Yang H, Lu L, Peng J, 2014a. Technical feasibility study on a standalone hybrid solar-wind system with pumped hydro storage for a remote island in Hong Kong. Renewable energy, 69: 7-15.
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[20] Ma T, Yang H, Lu L, Peng J, 2015. Pumped storage-based standalone photovoltaic power generation system: modeling and techno-economic optimization. Applied energy, 137: 649-659.
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[22] Internet: http://www.evergreen-marine.com/tei1/jsp/TEI1_Containers.jsp Accessed on 10.10.2016.
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[25] Dincer I, Rosen MA, 2015. Exergy Analysis of Heating, Refrigerating and Air Conditioning: Methods and Applications, Academic Press,
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[29] Ozcan H, Dincer I, 2013. Thermodynamic analysis of an integrated sofc, solar orc and absorption chiller for tri‐generation applications. Fuel Cells, 13(5):781-793.
[30] Li YQ, Yang MM, Zhao L, Wang FJ, 2012. The research of wind-light complementary based on pumped storage power system. Advanced Materials Research, 354: 1132-1136
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[33] Esfahani IJ, Yoo C, 2014. A highly efficient combined multi-effect evaporation-absorption heat pump and vapor-compression refrigeration part 2: Thermoeconomic and flexibility analysis. Energy, 75:327-37.
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[35] Internet: http://www.teamlandmark.com/water_storage/composite_elevated_tank/cost_advantage Accessed on 12.12.2016
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262-70.
20
FIGURE CAPTIONS Figure 1. Schematics of the standalone system. Figure 2. Schematics of the S–H2–VCR system. Figure 3. Schematics of the S–PHS–VCR system. Figure 4. Change in exergy efficiency at varying solar irradiation values for the system without storage. Figure 5. Exergy destruction rates at varying solar irradiation for the system without storage. Figure 6. Change in overpotential by current density in the PEMFC unit. Figure 7. Effects of fuel utilization ratio and PEMFC cell temperature on required H2 tank volume. Figure 8. Effect of fuel utilization ratio on PEMFC efficiencies and PEME power requirement. Figure 9. Effect of reservoir height on (a) storage efficiency, pump power requirement and (b) required PV panel Figure 10. Change in system efficiencies at months of a year in Aleppo for (a) S-H2-VCR and (b) S-PHS-VCR Figure 11. Shares of component costs for (a) the standalone system, (b) S-H2-VCR system, and (c) S-PHS-VCR system.
21
FIGURES WITH CAPTIONS
Figure 1. Schematics of the standalone system.
Figure 2. Schematics of the S–H2–VCR system. 22
Figure 3. Schematics of the S–PHS–VCR system. 0.2 R152a R134a R1234yf R600a R410a
Overall exergy efficiency (-)
0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 200
300
400
500
600
700
800
900
1000 1100 1200
Solar radiation (W/m2) Figure 4. Change in exergy efficiency at varying solar irradiation values for the system without storage.
23
20
Exergy destruction (kW)
18 R600a
16
R152a R134a R1234yf
14 12 10 8 6 4 200
400
600
800
1000
1200
Solar radiation(W/m2)
Figure 5. Exergy destruction rates at varying solar irradiation for the system without storage. 0.75
0.11 0.1
0.7 0.09 0.08
0.65
0.07 0.6
0.06 Eohm
0.55
Econc
0.5 50
100
150
200
250
300
350
400
0.05 0.04 0.03 450
Current Density (A/m2) Figure 6. Change in overpotential by current density in the PEMFC unit.
24
Overpotential(V)
Overpotential (V)
Eact
1.46
Hydrogen tank volume (m 3)
1.42 Uf =0.8
1.38
Uf =0.9 Uf =0.99
1.34 1.3 1.26 1.22 1.18 1.14 1.1 1.06 320
330
340
350
360
370
FC cell temperature (K) Figure 7. Effects of fuel utilization ratio and PEMFC cell temperature on required H2 tank volume. 0.4 W elec ψFC
18
0.35
16
0.3 ηFC
14
0.25
12
0.2
10 0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
FC efficiency (-)
Storage power requirement (kW)
20
0.15 1
Fuel utilization ratio (-) Figure 8. Effect of fuel utilization ratio on PEMFC efficiencies and PEME power requirement.
25
0.8
Storage Efficiency (-)
3.6 0.7
3.4 3.2
0.6
3 2.8
0.5 2
4
6
8
Pump power requirement (kW)
3.8
2.6 12
10
Reservoir height (m)
(a) 50
24
20 40
18 16
35
Reservoir diameter (m)
Required PV area (m2)
22 45
14 30 0
2
4
6
8
10
12 12
Reservoir height (m)
(b) Figure 9. Effect of reservoir height on (a) storage efficiency, pump power requirement and (b) required PV panel area.
26
(a)
(b) Figure 10. Change in system efficiencies at months of a year in Aleppo for (a) S-H2-VCR and (b) S-PHS-VCR systems.
27
(a)
(b)
(c) Figure 11. Shares of component costs for (a) the standalone system, (b) S-H2-VCR system, and (c) S-PHS-VCR system.
28
TABLE CAPTIONS Table 1. Storage conditions of some selected vaccines [4] Table 2. Input parameters and assumptions for thermodynamic analysis. Table 3. Cost correlations for the studied systems’ components. Table 4. Component and system cost results
29
TABLES WITH CAPTIONS Table 1. Storage conditions of some selected vaccines [4] Central Storage (up to 6 months)
Temporary storage (1 week – 3 months)
Medical Supply Storage temperature (°C) Oral polio
-15 - -25
Rubella
Can either be frozen or conditioned at 2°C
Tuberculosis Diphtheria Tetanus +2 - +8
Pertussis Hepatitis +2 - +8
Rabies Rabies antiserum Snake antiserum Scorpion antiserum
Table 2. Input parameters and assumptions for thermodynamic analysis. Parameter
Symbol
Unit
Range of variation
Refrigerant
-
-
R134a, R600a, R12, R22, R290, R410a, R152a, R1234yf
Reference temperature
T0
K
275-323
Reference pressure
P0
kPa
100
Evaporator temperature
Teva
K
275-281
Pressure ratio
BO
-
2-4
Compressor efficiency
ηis
-
0,85
Cooling load
Qeva
kW
11
Open cell voltage
Voc
V
0,2-0,8
30
Current density
J
A/m2
100-400
Limiting current density
Jlim
A/m2
2000
Fuel utilization factor
Uf
-
0,60-0,99
FC temperature
TFC
K
323-363
FC membrane thickness
σ
mm
0,18
Anode pressure
Pa
kPa
400
Transfer coefficient (anode)
α1
-
0,5
Transfer coefficient (cathode)
α2
-
1,0
Water activity
a
-
1
Number of electrons
z
-
2
Faraday constant
F
C/mol
96490
Gas constant
R
J/mol K
8,314
Concentration overpotential
γ
-
2
AC/DC inverter efficiency
ηAC/DC
-
0,95
Water chemical activity
aW
-
0,99
Hydrogen higher heating value
HHVH2
MJ/kg
283,760
PEME – PEMFC – H2
-
kPa
2500
Pump efficiency
ηpump
-
0,8
Hydraulic efficiency
ηhyd
-
0,9
Turbine efficiency
ηturb
-
0,86
Reservoir height
H
m
2-10
Water density
ρw
kg/m3
1000
Solar radiation
IT
W/m2
300-1100
factor
tank pressure
31
Table 3. Cost correlations for the studied systems’ components (in USD). Component
Cost Equation
Explanation
Ref.
Area is calculated based on Q Evaporator
309,14 ,
A U ΔT , where ΔT is 5°C and
[33]
U is 150 W/m2K Area is calculated based on Condenser
Δ , where ΔT is
C 516,62 A,
[33]
15°C and U is 200 W/m2K Expansion Valve
PR: Pressure ratio through the
37 0,68
[33] compressor between 2-4
Compressor
PV Panels
906,5
0,9
! "#$%
Vc: Compressor inlet specific volume in [33] m3/kg Apv: Panel area. Varies with the studied
310
[34] systems.
PEM-E
& : Required power to produce
, 940 &
[36]
adequate amount of hydrogen
PEM-FC
& : Generated power from the fuel
2400 &
[36]
cell. H2 Tank Hydraulic Pump Hydraulic Turbine Higher reservoir
240 'మ () 705& .
!
*1 +
SF: Safety factor (1.1)
0.2 1
"# $25.698&.$%&& .'.!( ( %/1.05 ") 89.46
32
-
[18-21]
h: reservoir height
[18-21]
V: Reservoir volume
[35]
Table 4. Component and system cost results Component
Cost
Compressor
$310,70
Condenser
$1245,00
Expansion Vane
$75,48
Evaporator
$3031,00
PEME
$10462,00
PEMFC
$5052,00
H2 Tank
$599,00
Hydraulic Pump
$3381,00
Hydraulic Turbine
$561,90
Higher Reservoir
$177319,00
İnverter (2300 W)
290 $
İnverter (3500 W)
450 $
PV Panel (System I)
$4901,00
PV Panel (System II)
$32333,00
PV Panel (System III)
$13640,00
Total Cost (System I)
$10840,00
Total Cost (System II)
$58738,00
Total Cost (System III)
$220000,00
33
Research Highlights
•
Solar based refrigeration systems with energy storage are proposed
•
Thermodynamic and economic assessments are applied
•
Cost of the pumped-hydro storage quadruples the hydrogen storage option
•
A case study is made for the city of Aleppo in Syria as an emergency region
34
S1359-4311(16)34522-7 http://dx.doi.org/10.1016/j.applthermaleng.2017.03.046 ATE 10050
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
31 December 2016 6 March 2017 10 March 2017
Please cite this article as: H. Ozcan, U.D. Akyavuz, Thermodynamic and economic assessment of off-grid portable cooling systems with energy storage for emergency areas, Applied Thermal Engineering (2017), doi: http:// dx.doi.org/10.1016/j.applthermaleng.2017.03.046
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
THERMODYNAMIC AND ECONOMIC ASSESSMENT OF OFF-GRID PORTABLE COOLING SYSTEMS WITH ENERGY STORAGE FOR EMERGENCY AREAS
1*
1
Hasan Ozcan, 2 Umit Deniz Akyavuz
Karabuk University, Engineering Faculty, Mechanical Engineering Department, Baliklar Kayasi Mevkii Demir Celik Kampusu, Karabuk, 78050, Turkey
2
Kilis 7 Aralık University, Kilis Vocational College, Electricity and Energy Department, Kilis, 79000, Turkey * E-mail: [email protected]
ABSTRACT This study aims to investigate performance and cost aspects of a solar powered portable cooling system to conserve first aid supplies for off-grid areas with energy storage. Due to the intermittent nature of solar energy availability, two energy storage options are considered for a stationary system. Additional to the standalone system without energy storage, hydrogen is selected to be the storage medium by considering electrolysis at day time, and use of a hydrogen fuel cell unit at night time. This system consists of solar photovoltaic cells, a Polymer Exchange Membrane (PEM) electrolysis unit (PEME), hydrogen tank, a PEM fuel cell unit (PEMFC), and a vapor compression refrigeration (VCR) system to condition a container rated with ~11 kW cooling load. The second system utilizes pumped – hydro storage (PHS) technology using a simple pump – turbine couple by storing water at a higher reservoir during day time and utilizing it to produce hydro power at night. Existence of higher reservoir brings a significant additional cost for the PHS system, making this configuration almost four times more costly than that of the hydrogen storage option, even though the storage efficiency of the PHS system is significantly higher than the hydrogen storage.
Keywords: Emergency areas, Syria, PEM electrolyser, PEM fuel cell, pumped-hydro storage, air conditioning,
1. Introduction Poverty, scarcity, starvation, infectious and epidemical diseases, and wars interrupt daily routines of communities which cause physical, economical, and social losses due to unstable natural or human based disasters [1]. It is of importance and crucial to have ready to act supplies during and after any emergency situation, where the primary services are first aid, feeding, and electricity, respectively [2]. For regions where
1
access to electricity is impossible, either a fast transportation of medical supplies are required, or a conditioned storage medium should be present for fast intervention to emergency situations. Storage conditions of medical and pharmaceutical supplies are specific to many factors. Most medical supplies have to be conditioned in a cold chain, to protect the products from losing their curing properties. Especially vaccines should not be interacted to undesired temperatures longer than an hour [3]. Temperature ranges of most vaccines and other medical supplies are suggested to be kept between 2°C - 8°C [4]. Storage conditions of some vaccines are provided in Table 1.
Refrigeration systems require a main source of electrical work in order to energize the system compressor for pressurized working fluid to complete the cyclic process. Solar energy is one of the best solutions for off-grid energy supply with its abundancy in many regions in the world, while solar photovoltaics (PV) has been one of the most promising renewable options to generate direct electricity at off-grid regions, which can be simply defined as a cell produced by thin semi-conductive junctions [5], leading to generation of direct electricity by many parallel or serial connected cells. The intermittent nature of solar energy brings the problem of steady energy generation which is generally solved by battery storage [6]. The most efficient battery storage technology is the Li-Ion storage with high efficiencies. Several researchers have imposed on specifically the environmental concerns on production and disposal of batteries, as well as their low life cycles, and daily self-discharging problems [7].
As alternatives to battery packs, electrolyser – fuel cell units are extensively under development stage. These storage systems show superiority than battery systems in many aspects, however, low round efficiency and high capital costs due to their industrial immaturity make this option a less feasible one. Cleanliness of both systems are also under further investigation [8]. Another option for energy storage can be considered as pumped-hydro storage, which is based on the simple idea of pumping water at a higher reservoir when there is available electricity, and utilizing the potential energy using a hydraulic turbine at night time [9].
Studies on PV based hydrogen production is comprehensively reviewed by Abdin et al, resulting in proposed PV based hydrogen storage systems with promising potentials, even though a high capital cost is required [10]. Other studies are performed by Aris and Shabani, on sustainability of PV-fuel cell couples [11], and Chowdhury and Moursed, on off-grid solar houses, their concepts, and long term advantages [12]. A portable solar PVdesalination system is recently introduced by Beitelmal and Fabris [13]. Many other studies on thermodynamic and economic aspects of such configurations can be found elsewhere [14-17].
2
Mathematical modeling and working principles of a PHS system using a wind-solar hybrid energy as the source is proposed by Ma et al, claiming that a 100% energy independency is attainable with such system configuration [18]. In another study a cost optimization of the aforementioned PHS system is performed using Genetic algorithm as an optimization tool by taking into account the number of PV modules, pump and reservoir dimensions [19]. Safety and economic criteria of a designed PHS system for a remote island are discussed by Ma et al for the main components and optimization comparsions [20]. A comprehensive review of several applied PHS systems and their feasibilities are reported in a recent literature survey performed by Sontake and Kalamkar [21]
In this study, a solar PV based refrigeration system with hydrogen storage and pumped-hydro storage are considered for a fixed Evergreen cooling container sized with a ~11 kW cooling load [22]; a comparative thermodynamic analysis is performed with parametric optimization, a case study is conducted by considering the solar data for Aleppo as an emergency region [23], and a rough estimation on system costs is made by considering the thermodynamic data obtained for system components. Studied systems with storage are also compared with the conventional solar VCR system in order to discuss on the thermodynamic and cost aspects of energy storage to provide a mobile and stationary system.
2. Systems Description A standalone solar VCR system as represented in Fig 1 is initially analyzed as the reference model. PV panels are used as the DC electricity supplying source for the compressor of the VCR system, where inverters are used to convert DC into AC. DC powered cooling systems are also available that directly utilizes the DC in the system compressor, resulting in enhanced cooling system performances. However, this study focuses on more conventional cooling systems with compressors driven by AC electricity. Therefore, inverters are present fro both PV panels and the PEMFC unit. Evaporator temperature is taken as a constant input parameter considering the medical supplies’ conservation temperature range, where the condensation temperature is based on the pressure ratio assumption made for the cycle. Here, a down selection process for the working fluid is also taken into account by making comparison for various refrigerants that match with the cooling temperature required for medical supply conservation. For this purpose, R134a, R152a, R600a, R12, R22, R290, R410a and R1234yf refrigerants are selected as candidate refrigerants, the highest efficient is selected, and adapted to other configurations with energy storage to provide performance enhanced systems.
3
The second system (S-H2-VCR) consists of solar PV panels connected to the VCR system along with the electrolysis unit. Electricity produced from PV panels are used for both the VCR and electrolyser unit at day time, while the generated hydrogen from the PEME is stored in a hydrogen tank to be utilized in the PEMFC unit when PV panels do not produce electricity. Here, oxygen is also considered to be stored to be a by-product which might also be useful as a medical consumable. This configuration makes the solar VCR a steadily working system. Schematics of the system is provided in Fig 2. The third system (S-PHS-VCR) differs from the second one in terms of the energy storage method. Here, instead of using an electrolyser – fuel cell couple, a hydraulic pump-turbine couple are considered to supply water to a higher reservoir to store it to be utilized at night time to produce power as shown in Fig 3. Here, generated electricity from PV panels is now both provided to the VCR and the hydraulic pump unit to provide potential energy to the water which is to be stored at a higher spheroid reservoir. This simple method of energy storage is expected to be high efficient with it maturity, however, it is also expected that the large sizes of required reservoirs might jeopardize the portability of the solar VCR unit.
3. Analyses and Assessment In this section, thermodynamic and economic analyses of the studied systems are represented with necessary assumptions and input parameters in subsections. 3.1. VCR subsystem Compressor power requirement, evaporator cooling load, and condenser heat release are defined by writing energy balances for individual components of the VCR subsystem as follows:
(1)
Where W and Q stand for work and heat. The thermal exergy of the container is
(2)
Finally the energetic and exergetic coefficient of performances (COP) of the VCR system are
(3)
4
3.2. Solar PV subsystem The load current is defined related to the voltage at constant temperature and solar radiation as follows [24]: / 1
(4)
Here, Io is the dark current, e is the electron load, k is the Boltzmann constant and T is the absolute PV cell temperature in Kelvins. Rearranging by equating the Eq (4) to zero, the definition becomes
/ 1 or
1
(5)
Here Voc is the open circuit voltage. The power in the PV cell is
exp 1"
(6)
The maximum power is obtained by equating the derivative of Eq (5) to zero as: 1
1
(7)
Where Vm is the voltage at the maximum power. Rearranging Eqs (4) and (7) one obtains the load current corresponding to the unit cell area as: /
/#
/
/#
(8)
and the maximum power per unit area as; /#
! "
/#
(9)
Open and short circuits are extreme cases and the power delivered to the load is zero. The power is at its peak at only for a specific load, which is dependent on the optimal current density and the cell voltage. The maximum energy efficiency attainable from the PV panel is the ratio of the maximum power delivered to the total solar energy input $
#
(10)
Where In is the total solar irradiation delivered on the total PV surface (In=IA*Ac). The solar exergy input to the PV panels are as follows [37]: $ 1
" #
(11)
Here, % is the reference temperature while & is the sun temperature taken is 5700 K [25]. Since this equation involves many peculiarities of thermal radiation such as irreversibility of emission and absorption of radiation, significance of thermal radiation exergy for a zero environment temperature, exergy at a varying environment temperature, and the analogy between exergy of substance and radiation as mentioned in [37], it is selected to be
5
the main solar exergy input for the present study. The exergy efficiency of the PV subsystem is the ratio of the maximum electricity produced to the solar exergy input as follows: %
''
(12)
3.3. Hydrogen Storage Subsystem 3.3.1 Electrolyser The thermo-neutral voltage transfers the reaction enthalpy to the adiabatic cell and defined as follows [26]: )
∆+
(13)
'
The reversible cell voltage (Ecell) is the potential applied on the cell and it is the required free energy to conduct the electrochemical reaction and can be explained by the Nernst equation. In this equation, the standard cell voltage is (E0) is 1.229 V, the equilibrium constant is &, '+ -%./ (/')+ - %./ (. The reversible cell voltage is
∆1
,$
'
2 '
ln &,
(14)
Current density of the electrolysis cell is the ratio of the current to the cell area and dependent on the activation energy '∆. ,$ 20 ,$ ( based on the Butler-Volmer equation 3 3
exp
4' , 2
exp
54 ' , 2
(15)
Here, 1 is the transfer coefficient varying from 0 to 1, and the exchange current density 2% is 2% 304
' , 2
(16)
Total electrolysis voltage ( ,$ ) is the summation of the reversible cell voltage along with the activation and concentration overpotentials ,$ ,$ ,$ ,$ ),$
(17)
The activation, concentration and ohmic overpotentials are defined as follows: 2
3
'
3
,$ , at 2 5 2% (18) 3
,$ 6 , at 2 7 2% 3
,$
5
2
8
6'
9
(19)
9
),$ 2 :%
(20)
78
6
Where b is the Tafel constant, JL is the limiting current density, ; is ionic conductivity through cell, and < is water content ar the membrane-electrode interface. Finally, energy and exergy efficiencies of this unit can be defined by considering the higher heating value (HHV) of hydrogen produced as the useful output $
:.;:
(21)
,
Here there is a constant relationship between the exergy content of hydrogen and it is heating value, therefore, a relation can be made between the exergy and the energy efficiencies as follows: % 0.83$
(22)
3.3.2 Fuel Cell For the PEM fuel cell, the reversible voltage across the load can be defined by taking the overpotentials into account as follows: $ )
(23)
Here, it is possible to express the voltage across the load (Eload) in terms of the reversible cell voltage and overpotentials through the cell as follows [27]: $ 1.229 8.5 105 298.15 4.3085 105/ ' + 0.5 - (
4 4 2 4 4
6'
3 <
2 D 3
4 4 2 4 4
6'
3 =>!5.?/
4 4 2 4 4
6'
3
3
"
%.%%/ ;@5%.%% :
(24)
Here, the first term is the open circuit voltage (1.229 V), second term is present for the effect of the cell temperature, the third term is correlated to represent the effects of both cell temperature and pressure, the remaining terms represent the activation, concentration, and the ohmic overpotentials, respectively. Here, it is assumed that the transfer coefficient is taken as 0.5 for the anode and 1 for the cathode, the number of charges are z=2, and the limiting current density is 2000 A/m2. Many other input parameters and their numerical values are provided in Table 2. The exchange current density of the cell is 2% 1.08 0.086A
(25)
Where Tb is the boundary temperature, the concentration overpotential exponent F is around 2, and the concentration overpotential factor and the membrane humidity factor are defined, respectively, as follows: DG
7.16 105 A 0.622 1.45 105 A 1.68 8.66 105 A 0.068 1.60 105 A 0.54
0 N ) N 1L 0.043 17.8) 39.85) 39.85) MG 14 1.4) 1
1I)N3
I 2 )JL K 2 )J
(26)
(27) 7
Here the activity coefficient of the water is ) O+ - / where P is the membrane pressure and O+ - is molar fraction of water. The cathode pressure is correlated using 8.52- equation. Assuming there is linear exchange from the molar fraction from anode to cathode, partial pressures of hydrogen and oxygen are given as (+ O+ B ) and (- O- C ), based on the anode and cathode pressures. It is possible to perform a molar fraction calculation for the fuel O+ , and oxidant O- as documented in [27]. The power output from the fuel cell stack is related to the fuel utilization factor (Uf), and amount of feed fuel ( ), as a product of actual voltage as follows: 'C 2 0 P $
(28)
Energy and exergy efficiencies of the fuel cell stack are defined as the ratios of useful outputs per inputs, respectively, as follows [28,29]: $'C %'C
(29)
9+
(30)
3.3.3 Hydrogen Tank Since it is expected that the fuel cell unit produces a constant amount of power to drive the VCR system compressor, it is possible to determine the amount of hydrogen required to run this process at night time. Here the required amount of hydrogen can be found using Eq (27), and the stored hydrogen is calculated using this input as follows: Q, 14 3600 + 2
(31)
Here the numbers 14, 3600, and 2 correspond to the nigh time hours, seconds of an hour, and the molecular weight of hydrogen, respectively. This simple equation allows to estimate the size of the hydrogen tank for storage, which is important to know for a portable system. The volume of the tank can be calculated by using the specific volume of hydrogen at its storage temperature and pressure as follows: Q, R
(32)
3.4. PHS system Conversion of electrical - potential energy principle is the basics of the PHS system using two simple mechanisms to circulate water between higher and lower reservoirs. Through the analysis, coefficients for water pumping, S> and turbine power production, S are two important parameters, indicating the water volume stored by unit energy. [30, 31]. The suction volume flow rate of the pump from the lower reservoir is T> J
D E FG)
S> EH> J
(33) 8
Here $> is pump efficiency, U is water density, V is gravitational force, is the higher reservoir height, and EH> J is the power delivered from the PV panels. The turbine power is defined as: J $ UV T J S T J
(34)
where $ is turbine efficiency and, T J is the volume flow rate of water entering the turbine. The total water stored at the higher reservoir can be defined with its potential energy content as follows [32]: > , Gü $
D FG)
(35)
,:J%!
3.5. Economic Analysis Since the corresponding portable cooling system is specifically designed as a humanitarian application, a rough cost estimation is considered by using the thermodynamic and cost data of all components throughout the systems studied. Table 3 summarizes correlations for component costs with additional comments on individual equations. Total system costs are the summation of component cost with a 1.1 of auxiliary factor and defined for each system as follows: $,KLK E #PW
(36)
$,KLK E $ 'C #PW
(37)
$,KLK 'E > & ( #PW
(38)
All aforementioned cost parameters are updated using the CEPCI (Chemical Engineering Plant Cost Index) in order to provide up-to-date component costs.
4. Results and Discussion 4.1. Thermodynamic analysis results In this study, a thermodynamic analysis and economic assessment of three solar cooling applications are comparatively performed. Parametric optimization of all three systems are conducted using Engineering Equation Solver (EES) is utilized for analyses. Here, many input parameters as represented in Table 2 are used as variables for optimized system configurations. Results of thermodynamic and electrochemical models used for analysis are manipulated using various input data in order to accomplish higher performing and lower cost systems. Various representations of parametric optimizations are given in this section, visualizing the changes in system performances by varying several input parameters.
An initial modeling of the system is performed without considering the storage option in order to parametrically optimize the refrigeration system by taking into account many refrigerants showing similar thermophysical 9
properties as R134a. This working fluid is compared to R600, R1234yf, R600a, and R152 fluids. A down selection is made by eliminating R410a due to low exergy efficiency values of the overall system compared to those of other considered fluids as represented in Fig 4. Total exergy destruction rate shows its lowest value for the R152a fluid, which is slightly lower than R134a, while other selected fluids show higher exergy destruction rates at same conditions as shown in Fig 5. It is also obvious that the system tends to show lower exergy efficiencies at higher solar radiation. This due to the definition of solar exergy input.
R152a is selected as the working fluid for the systems with storage, in which the energetic COP values are 5.99 for ideal conditions. For the 11 kW container, the power requirement of the compressor is 2 kW. Therefore, sequence of the modeling is initiated with the PEMFC stack with a known power output. The current density affects all considered overpotentials through the FC with a similar trend as shown in Fig. 6. Increased current density leads to higher overpotentials, however, increase in overpotentials show a decreasing trend leading to higher FC voltage (~0.49 V). Since the required power is known, it is possible to determine the required molar hydrogen rate for the FC stack. 0.022 moles/s of H2 is required to produce enough power to run the refrigeration system. Energy and exergy efficiencies of the FC stack are determined to be 33%, and 39.6%, respectively.
Considering the required molar flow rate of H2, one can determine the total daily H2 production requirement of the electrolysis cell. For 10 hours of sunbathing 2.27 kg of H2 should be stored in daily basis. Since both FC and electrolysis cells assumed to work at same pressure to prevent from losses due to expansion or compression of gas agents, H2 storage is considered to be at the same pressure with both cells, namely 2500 kPa, and at this pressure level, specific volume of H2 is ~0.5 m3/kg. This brings a H2 tank volume requirement of 1.15 m3, which is in a feasible range for the considered container cooling load. A parametric study is conducted in Fig. 7 to evaluate effects of fuel utilization factor and cell temperature of the FC stack on the hydrogen tank volume requirement. Higher utilization ratio and cell temperature values result in higher tank volumes, even though these parameters are favorable for higher FC stack energy and exergy performances. H2 discharge lasts in 14 hours, however, charging of H2 should be made in 10 hours of sunbathing while PV system produces electricity. Therefore, produced hydrogen rate should be higher than the consumed hydrogen at a rate of 0.0315 mole/s. In order to produce this amount, the power requirement of the electrolysis unit is determined to be 11.13 kW. Another parametric optimization is conducted for a decreased power requirement of the electrolysis cell by
10
manipulating the FC fuel utilization factor as in Fig. 8. Higher fuel utilization factor significantly affects the power requirement of the electrolysis unit, due to FC H2 requirement significantly decreases.
Using the simple definitions for a rough model of the PHS system, the higher reservoir conditions should be taken into account for a parametric optimization. Reservoir height and the spherical type reservoir dimeter and volume are the primary cost influencing parameters. However, change in these variables also affect the storage efficiency and the required solar panel area to provide power for the hydraulic pump. Higher reservoir height results in decreased storage efficiency and increased pump power requirement, and higher amount solar PV panels as represented in Figs 9a and 9b. Even though higher height does not seem to be feasible both thermodynamically and economically, a higher volume of the reservoir would bring a very bulky component, which possibly jeopardize the portability of the system. Considering the reservoir height as a variable input, the total PHS efficiency shows a significant energy and exergy efficiency value of 74% at the lowest height value, while decreasing down to 54% at higher reservoir height. It should also be noted that, since the useful output and required input of the PHS system are related to power, the energy and exergy efficiencies of the PHS system are equal.
The PV panels are required to supply enough power for both the electrolysis unit and the refrigeration system, Therefore sizing of the panel area is based on the sum of these power requirements. The total PV area required to run this S-H2-VCR system is found to be around 105 m2, for the PHS system required PV area is around 46 m2, while this value is only 16.7 m2 for the standalone system without storage. Energy and exergy efficiencies of the PV panels are determined to be 25.2% and 27.5% respectively, consistent for all configurations. Considering the overall system performances for the standalone system, S-H2-VCR, and S-PHS-VCR systems, energetic COP values of studied systems are 1.39, 0.211, and 1.34, while exergy efficiencies are 11.54%, 1.75%, and 11.15%, respectively. Low values for the second system are due to the definition of overall system efficiencies in which generated H2 is not considered as an output from the system.
Considering the monthly solar irradiation data [23] for Aleppo-Syria as an emergency region COP and exergy efficiency values at each month of the year are provided in Figs 10a and 10b. Since the solar irradiation value is the major input for both systems, months with lower irradiations provide the highest efficiencies. With the two major assumptions made for this city, namely ten hours of sunbathing which corresponds to the lowest daylight
11
time during the year in Aleppo, and the lowest possible irradiation values, both systems with storage might safely produce conditioned air for medical supplies throughout the year.
4.2. Economic analysis results Table 4 summarizes all component costs of all three systems considered. The highest cost belongs to the higher reservoir of the S-PHS-VCR system, corresponding to almost ~80% of the total system cost. Since the required panel area changes for every system, costs are also variable. Share of solar panel cost shows its highest value for the S-H2-VCR system due to higher solar panel area requirement for H2 production during day time, occupying around 60% of the total cost of this system. Cost shares of components of each studied system are represented in Figs 11a, 11b and 11c. Even though the thermodynamic performance of the S-PHS-VCR system is significantly higher than that of the S-H2-VCR system, high reservoir cost and a predicted bulky size of the reservoir make such system costly while jeopardizing the portability of such system. However, it might still be possible to transport such reservoir for this 11 kW system considered. Higher PV area requirement of the S-H2-VCR system might also be considered as a problem for a portable system, however, recent technologies for light, foldable, and cheaper PV cell design make this configuration a promising one for a feasible stationary and portable cooling container.
Comparing both systems with the battery storage technology, a simple assessment can be made for battery costs with their life cycles. For the 2 kW peak-power requiring air conditioning system, and 16 hours of stored power utilization requirement at night, the total energy to be storage by the battery packages are to be 32 kW/h. In order to possess a reasonable life cycle, the discharge rate of each battery should not be lower than 50%, which may increase the life cycle of a battery package up to 6 years. Therefore, this simple approach suggests at least a 64 kW/h battery storage for the studied system [38]. Considering the commonly used Li-ion battery systems, updated costs are around 150 kW/h, cost of the battery storage is roughly expected to be $9600. Even though low life cycles, maintenance requirements, and possible hazardous disposal of these battery systems are seems to be infeasible, its cost is significantly lower than that of the PHS and similar to the S-H2-VCR system.
5. Conclusions
12
Present study thermodynamically and economically evaluates a solar based refrigeration system with hydrogen and pumped-hydro storage to keep medical supplies at their desired temperature ranges for off-grid areas with a case for Aleppo-Syria. The main motivation is to store medical supplies at their desired storage temperatures. Most medical supplies, especially vaccines have to be conditioned in a cold chain, to protect supplies from losing their curing properties. If vaccines are interacted to the environmental temperatures more than an hour, the weak viral organisms may either die or get strong, making both situations hazardous for human body functioning when injected. The reported energy and exergy efficiencies, as well as the rough cost estimation of studied systems are in feasible ranges and all studied sub-systems relatively possess high life cycles with acceptable maturity. Hydrogen storage shows superiority with low cost and a light - small size storage capability than that of the pumped-hydro storage option, while the pumped-hydro storage shows significantly higher energy and exergy performances. Estimated cost of the PHS option almost quadruples the H2 storage option, even though performance characteristics of PHS are significantly higher. Battery storage can also be another option with possible low life cycles and high round efficiencies. Such configurations can conveniently be used for off-grid areas, not only for cooling, also for continuous electricity supply with mature and on-the-shelf equipment.
6. Acknowledgement Authors acknowledge the financial support from the scientific research projects unit of Karabuk University under the project number KBÜ-BAP-16/2-YL-091.
Nomenclature A
: Area
a
: Membrane water activity
C
: Cost
E
: Voltage
Ex
: Exergy
e
: Electron load
F
: Faraday constant
h
: Enthalpy
h, z
: height, charge
I
: Current
13
IT
: Solar irradiation
J
: Current density
k
: Boltzmann constant
P
: Pressure, power
Q
: Thermal energy
R
: Resistance, Universal gas constant
s
:Entropy
SF
: Safety factor
U
: Utilization, heat exchange factor
V
: Velocity
W
: Work
y
: molar fraction
Greek Symbols ??
: Exponent
η
: Energy efficiency
ψ
: Exergy efficiency
σ
: Membrane thickness
α
: Load transfer coefficient
γ
: Exponent for concentration overpotential
γ
: Molar fraction
β
: Facto for concentration overpotential
ω
: Membrane humidity factor
ρ
: Density
ΔT
: Temperature difference
Subscripts and Superscripts a
: Anode
act
: Activation
b
: Boundary
14
c
: Compressor
cat
: Cathode
cell
: Cell
ch
: Chemical
conc
: Concentration
cond
: Condenser
cont
: Container
D
: Destruction
elec
: Electrolyser
en
: Energetic
eva
: Evaporator
ex
: Exergetic
f
: fuel
gen
: generation
ev
: expansion valve
in
: inlet
is
: Isentropic
kin
: Kinetic
L
: Limit
m, max
: Maximum
nf
: Non-flow
o
: Reference
oc
: Open circuit
ohm
: Resistance
out
: output
ph
: Physical
pot
: Potential
pump
: Pump
ref
: Refrigerant
rev
: Reversible
15
res
: Reservoir
sat
: Saturation
t
: Turbine
tot
: Total
w
: water
Acronyms AC
: Alternative Current
CEPCI
: Chemical engineering plant cost index
DC
: Direct Current
PEME
: Polymer exchange membrane electrolysis
PEMFC
: Polymer exchange membrane fuel cell
PHS
: Pumped-hydro storage
PR
: Pressure ratio
PV
: Photovoltaics
S
: Solar
VCR
: Vapor compression refrigeration
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[25] Dincer I, Rosen MA, 2015. Exergy Analysis of Heating, Refrigerating and Air Conditioning: Methods and Applications, Academic Press,
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262-70.
20
FIGURE CAPTIONS Figure 1. Schematics of the standalone system. Figure 2. Schematics of the S–H2–VCR system. Figure 3. Schematics of the S–PHS–VCR system. Figure 4. Change in exergy efficiency at varying solar irradiation values for the system without storage. Figure 5. Exergy destruction rates at varying solar irradiation for the system without storage. Figure 6. Change in overpotential by current density in the PEMFC unit. Figure 7. Effects of fuel utilization ratio and PEMFC cell temperature on required H2 tank volume. Figure 8. Effect of fuel utilization ratio on PEMFC efficiencies and PEME power requirement. Figure 9. Effect of reservoir height on (a) storage efficiency, pump power requirement and (b) required PV panel Figure 10. Change in system efficiencies at months of a year in Aleppo for (a) S-H2-VCR and (b) S-PHS-VCR Figure 11. Shares of component costs for (a) the standalone system, (b) S-H2-VCR system, and (c) S-PHS-VCR system.
21
FIGURES WITH CAPTIONS
Figure 1. Schematics of the standalone system.
Figure 2. Schematics of the S–H2–VCR system. 22
Figure 3. Schematics of the S–PHS–VCR system. 0.2 R152a R134a R1234yf R600a R410a
Overall exergy efficiency (-)
0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 200
300
400
500
600
700
800
900
1000 1100 1200
Solar radiation (W/m2) Figure 4. Change in exergy efficiency at varying solar irradiation values for the system without storage.
23
20
Exergy destruction (kW)
18 R600a
16
R152a R134a R1234yf
14 12 10 8 6 4 200
400
600
800
1000
1200
Solar radiation(W/m2)
Figure 5. Exergy destruction rates at varying solar irradiation for the system without storage. 0.75
0.11 0.1
0.7 0.09 0.08
0.65
0.07 0.6
0.06 Eohm
0.55
Econc
0.5 50
100
150
200
250
300
350
400
0.05 0.04 0.03 450
Current Density (A/m2) Figure 6. Change in overpotential by current density in the PEMFC unit.
24
Overpotential(V)
Overpotential (V)
Eact
1.46
Hydrogen tank volume (m 3)
1.42 Uf =0.8
1.38
Uf =0.9 Uf =0.99
1.34 1.3 1.26 1.22 1.18 1.14 1.1 1.06 320
330
340
350
360
370
FC cell temperature (K) Figure 7. Effects of fuel utilization ratio and PEMFC cell temperature on required H2 tank volume. 0.4 W elec ψFC
18
0.35
16
0.3 ηFC
14
0.25
12
0.2
10 0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
FC efficiency (-)
Storage power requirement (kW)
20
0.15 1
Fuel utilization ratio (-) Figure 8. Effect of fuel utilization ratio on PEMFC efficiencies and PEME power requirement.
25
0.8
Storage Efficiency (-)
3.6 0.7
3.4 3.2
0.6
3 2.8
0.5 2
4
6
8
Pump power requirement (kW)
3.8
2.6 12
10
Reservoir height (m)
(a) 50
24
20 40
18 16
35
Reservoir diameter (m)
Required PV area (m2)
22 45
14 30 0
2
4
6
8
10
12 12
Reservoir height (m)
(b) Figure 9. Effect of reservoir height on (a) storage efficiency, pump power requirement and (b) required PV panel area.
26
(a)
(b) Figure 10. Change in system efficiencies at months of a year in Aleppo for (a) S-H2-VCR and (b) S-PHS-VCR systems.
27
(a)
(b)
(c) Figure 11. Shares of component costs for (a) the standalone system, (b) S-H2-VCR system, and (c) S-PHS-VCR system.
28
TABLE CAPTIONS Table 1. Storage conditions of some selected vaccines [4] Table 2. Input parameters and assumptions for thermodynamic analysis. Table 3. Cost correlations for the studied systems’ components. Table 4. Component and system cost results
29
TABLES WITH CAPTIONS Table 1. Storage conditions of some selected vaccines [4] Central Storage (up to 6 months)
Temporary storage (1 week – 3 months)
Medical Supply Storage temperature (°C) Oral polio
-15 - -25
Rubella
Can either be frozen or conditioned at 2°C
Tuberculosis Diphtheria Tetanus +2 - +8
Pertussis Hepatitis +2 - +8
Rabies Rabies antiserum Snake antiserum Scorpion antiserum
Table 2. Input parameters and assumptions for thermodynamic analysis. Parameter
Symbol
Unit
Range of variation
Refrigerant
-
-
R134a, R600a, R12, R22, R290, R410a, R152a, R1234yf
Reference temperature
T0
K
275-323
Reference pressure
P0
kPa
100
Evaporator temperature
Teva
K
275-281
Pressure ratio
BO
-
2-4
Compressor efficiency
ηis
-
0,85
Cooling load
Qeva
kW
11
Open cell voltage
Voc
V
0,2-0,8
30
Current density
J
A/m2
100-400
Limiting current density
Jlim
A/m2
2000
Fuel utilization factor
Uf
-
0,60-0,99
FC temperature
TFC
K
323-363
FC membrane thickness
σ
mm
0,18
Anode pressure
Pa
kPa
400
Transfer coefficient (anode)
α1
-
0,5
Transfer coefficient (cathode)
α2
-
1,0
Water activity
a
-
1
Number of electrons
z
-
2
Faraday constant
F
C/mol
96490
Gas constant
R
J/mol K
8,314
Concentration overpotential
γ
-
2
AC/DC inverter efficiency
ηAC/DC
-
0,95
Water chemical activity
aW
-
0,99
Hydrogen higher heating value
HHVH2
MJ/kg
283,760
PEME – PEMFC – H2
-
kPa
2500
Pump efficiency
ηpump
-
0,8
Hydraulic efficiency
ηhyd
-
0,9
Turbine efficiency
ηturb
-
0,86
Reservoir height
H
m
2-10
Water density
ρw
kg/m3
1000
Solar radiation
IT
W/m2
300-1100
factor
tank pressure
31
Table 3. Cost correlations for the studied systems’ components (in USD). Component
Cost Equation
Explanation
Ref.
Area is calculated based on Q Evaporator
309,14 ,
A U ΔT , where ΔT is 5°C and
[33]
U is 150 W/m2K Area is calculated based on Condenser
Δ , where ΔT is
C 516,62 A,
[33]
15°C and U is 200 W/m2K Expansion Valve
PR: Pressure ratio through the
37 0,68
[33] compressor between 2-4
Compressor
PV Panels
906,5
0,9
! "#$%
Vc: Compressor inlet specific volume in [33] m3/kg Apv: Panel area. Varies with the studied
310
[34] systems.
PEM-E
& : Required power to produce
, 940 &
[36]
adequate amount of hydrogen
PEM-FC
& : Generated power from the fuel
2400 &
[36]
cell. H2 Tank Hydraulic Pump Hydraulic Turbine Higher reservoir
240 'మ () 705& .
!
*1 +
SF: Safety factor (1.1)
0.2 1
"# $25.698&.$%&& .'.!( ( %/1.05 ") 89.46
32
-
[18-21]
h: reservoir height
[18-21]
V: Reservoir volume
[35]
Table 4. Component and system cost results Component
Cost
Compressor
$310,70
Condenser
$1245,00
Expansion Vane
$75,48
Evaporator
$3031,00
PEME
$10462,00
PEMFC
$5052,00
H2 Tank
$599,00
Hydraulic Pump
$3381,00
Hydraulic Turbine
$561,90
Higher Reservoir
$177319,00
İnverter (2300 W)
290 $
İnverter (3500 W)
450 $
PV Panel (System I)
$4901,00
PV Panel (System II)
$32333,00
PV Panel (System III)
$13640,00
Total Cost (System I)
$10840,00
Total Cost (System II)
$58738,00
Total Cost (System III)
$220000,00
33
Research Highlights
•
Solar based refrigeration systems with energy storage are proposed
•
Thermodynamic and economic assessments are applied
•
Cost of the pumped-hydro storage quadruples the hydrogen storage option
•
A case study is made for the city of Aleppo in Syria as an emergency region
34